Add support for chained row IDs.

Previously you couldn't do row definitions beyond a single level: 'row.postcode' would work, but not 'row.address.postcode'.
Add support for chained row IDs in 'exportable' directive.
Add primitive exporter support for XLS (2003).

The format is supported by later versions, but will show a prompt indicating that the file is not on the expected format and that the source should be supported.  Later versions might want to support the open XLSX standard instead.